1. Field of the Invention
The present invention pertains to indexable cutter inserts for a rotary cutter such as a ball end mill.
2. Prior Art
In a rotary cutter such as a ball end mill and a drill bit, the cutting edge undergoes a great cutting load at its leading end portion disposed adjacent to an axis of rotation of a cutter body since the cutting speed is relatively low in the vicinity of the axis of rotation. As a result, the cutting edge is susceptible to damage or chipping at its leading end portion. Accordingly, in recent years, various cutter inserts of such a type that the strength of the cutting edge has been enhanced by modifying the shape of the leading end portion have been developed. FIGS. 1 and 2 show one such conventional cutter insert as disclosed in Published Examined Japanese Patent Application No. 59-21725. The insert 100 comprises a plate-like body defined by a front face 102, a rear face 104 and a side periphery lying between the front and rear faces 102 and 104. A pair of opposite notches 106 of a generally V-shape are formed in the side periphery to interrupt the side periphery to provide a pair of opposite side faces 108. Each of the V-shaped notches 106 extends from the front face 102 to the rear face 104 and is defined by a pair of first and second opposed walls 110 and 112. The first wall 110 is flat and perpendicular to the front face 102 while the second wall 112 is convexly curved when viewed in a direction perpendicular to the first wall 110. Each of the side faces 108 assumes a generally quadrantal shape when viewed in a direction perpendicular to the front face 102, and cooperates with the front face 102 and the second wall 110 of the notch 106 to define a cutting edge 114 which is convexly arcuate as viewed in the same direction. A marginal portion of the front face 102 and the second wall 112 disposed adjacent to and extending along each cutting edge 114 serves as a rake surface. Each arcuate convex side face 108 serves as a flank for the cutting edge 114 and is sloping inwardly in a direction away from the front face 102, and thus the relief angle .alpha. for the flank is rendered positive.
As shown in FIGS. 3 to 5, the insert 100 is removably secured by a clamp screw 116 to a body 118 of the ball end mill with the rear face 104 mated with a recessed seat surface 120, in such a manner that an indexed one of the cutting edges 114 is so disposed as to extend outwardly beyond the front end and outer periphery of the end mill body 118, and that the leading end portion 114a of the indexed cutting edge 114 disposed adjacent to an axis O of the end mill body 118 is convexly curved when viewed axially of the end mill body 118. Besides, the axial rake angle .theta. for the indexed cutting edge 114 is rendered positive.
In the cutter insert described above, the leading end portion 114a of the indexed cutting edge 114 begins to engage a workpiece first, and then other portions of the cutting edge 114 begin to engage the workpiece as the end mill body 118 rotates. Accordingly, the cutting load increases gradually as the cutting operation proceeds, and a great load will not be exerted on the indexed cutting edge 114 at a time, so that the cutting edge 114 is less liable to breakage.
In such a prior art insert, however, since a positive relief angle .alpha. is imparted to each arcuate convex flank 108, it is impossible to define other cutting edges at the marginal ridges of the rear face 104, and therefore the insert is not economical. Besides, as shown in FIG. 2, an angle .beta. defined by a leading end of the second wall 112 and a straight line tangent to the cutting edge 114 at its leading end is less than 90.degree., and hence the strength of the cutting edge 114 is not sufficiently high at its leading end. In addition, the flat wall 112 of the V-shaped notch 106 prevents a smooth discharge of chips during the cutting operation and causes the jamming or wedging of chips, so that the cutting edge 114 is liable to damage.